1. Field of Invention
The present invention relates, in general, to the field of corrosion sensing and monitoring. More specifically, the present invention provides a system and a method to monitor the condition of a protective coating (e.g., paint) on a substrate and to detect degradation in the protectiveness of the coating prior to the onset of substrate corrosion.
2. Description of the Related Art
Atmospheric corrosion of steels, aluminum alloys, and Al-clad aluminum alloys is a problem for many civil engineering structures, commercial and military vehicles, and aircraft. Paint is usually the primary means to prevent the corrosion of steel bridge components, automobiles, trucks, and aircraft. Under ideal conditions, the coating provides a continuous layer that is impervious to moisture. The ability of a coating to provide protection is dependent on a variety of factors, including the composition of the coating, service application, and the service conditions such as chemistry of the environment, amount of UV exposure, temperature, and relative humidity. Another important factor that determines the protective quality of the coating is the extent of damage caused by impact and abrasion during service. At present, maintenance cycles for commercial and military aircraft and ground vehicles, as well as engineered structures, is based on experience and appearance rather than a quantitative determination of coating integrity. Limitations to experience, variations in exposure conditions and the possibility of extensive corrosion damage occurring in hidden areas such as seams and lap joints are serious drawbacks to the present methodology used to determine maintenance schedules and results in considerable unnecessary costs. For example, it has been reported that the U.S. Air Force spends an average of $800M annually on corrosion control measures, of which coating maintenance operations are the majority (Cooke et al., 1998).
Monitoring the corrosion of engineered structures and vehicles has been proposed using a variety of corrosion sensors. The level of complexity of the proposed sensors varies from simple designs where the magnitude of either a galvanic or an impressed current is monitored over time to more advanced designs that incorporate multiple miniaturized electrodes including reference, chloride, and pH electrodes. The simple galvanic sensor design is robust and has been shown to be effective as time of wetness sensors and can be used to assess the corrosivity of the environment. Sensors with integrated reference, pH, and chloride ion selective electrodes can be used to measure corrosion rate and collect more detailed information on the variations in environment chemistry. While the ability to determine the condition of an existing coating over time would be beneficial in determining maintenance needs and possibly assessing early coating damage or degradation and preventing corrosion of the vehicle or structure, the present suite of sensors designed to detect corrosion and marketed to predicting the lifetime of the engineered components are not useful for determining the condition of the protective paint coatings.
A number of further attempts to monitor or measure the corrosion of engineered structures and vehicles are described in the following U.S. patent references.
U.S. Pat. No. 5,746,905 issued to Murray describes an invention for evaluating an organic coating using electrochemical impedance spectroscopy (EIS). EIS has been used to examine organic coatings in the manner generally described in Murray for some time. The structures and methods described in Murray do not, however, contemplate embedding the sensor system under the coating. In addition, the Murray disclosure requires three electrodes, one of which must be connected to the substrate. Finally, the Murray disclosure requires the use of a cell having a conductive interface with the coating.
U.S. Pat. No. 5,698,085 issued to Yu describes an invention to evaluate coating properties with a main focus on metallic coatings that corrode. Yu calls for the use of a test cell that is filled with a solution and requires electrical contact with the substrate for measurement.
U.S. Pat. No. 6,208,128 issued to Braconnier et al. describes a method for monitoring the loss of a refractory coating in a blast furnace. The Braconnier et al. disclosure describes at least two electrodes embedded under the refractory coating to make electrical measurements. The described system is directed to determining the loss of thickness in the refractory coating over time as opposed to organic coating degradation/defect detection and corrosion. The Braconnier et al. invention cannot be applied or modified to accomplish this latter type of degradation detection and monitoring. In addition, the electrodes utilized in the Braconnier et al. disclosure are consumed over time which complicates the long term consistency of the system since the response of the system is dependent on the length of the electrodes.
U.S. Pat. No. 4,839,593 issued to Spies describes a method for detecting the presence of corrosion through changes in an induced current arising from an electromagnetic pulse. The Spies disclosure does use radio frequency communication for instrument control and data acquisition. The method of measurement, however, is quite distinct and cannot be modified to monitor the performance of coatings in the long term. Furthermore, the specific structure disclosed in Spies is not directly embeddable in the coating.
U.S. Pat. No. 3,357,237 issued to Le Bel describes a method for monitoring the loss of material at a surface using ablation. This invention utilizes embedded electrodes and carries out electrical measurements. However, the type of electrical measurement being made monitors the loss of material as opposed to degradation of a coating. Thus, the physical properties measured and analyzed by Le Bel are distinct from the measurements and properties intended in the present invention.
U.S. Pat. No. 3,782,181 issued to Gurtler describes an invention similar in many respects to the Le Bel disclosure identified above. Gurtler involves a surface ablation sensor that is embedded under the surface to monitor the loss of materials over time.
In general the prior art fails to solve many of the problems associated with the monitoring of coating effectiveness for such structures as metallic sheets covered over with paint. In many instances in the above patents the coatings that are contemplated are thicker pipe jackets and the like with the measurements being directed to changes in the overall coating thickness at various locations. In addition, many of the systems described in the prior art require an electrical connection to the substrate as a basis for making electrical measurements characterizing the coating. Many systems in the prior art additionally fail to lend themselves to installation after placement of the coating and would be difficult to retrofit into an existing substrate/coating structure.